1.5 The Lisp Interpreter

Based on what we have seen, we can now start to figure out what the
Lisp interpreter does when we command it to evaluate a list.
First, it looks to see whether there is a quote before the list; if
there is, the interpreter just gives us the list. On the other
hand, if there is no quote, the interpreter looks at the first element
in the list and sees whether it has a function definition. If it does,
the interpreter carries out the instructions in the function definition.
Otherwise, the interpreter prints an error message.

This is how Lisp works. Simple. There are added complications which we
will get to in a minute, but these are the fundamentals. Of course, to
write Lisp programs, you need to know how to write function definitions
and attach them to names, and how to do this without confusing either
yourself or the computer.